linux-kernel - Re: BUG: GCC-4.4.x changes the function frame on some functions

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Date:	Thu, 19 Nov 2009 14:25:01 -0700
From:	Jeff Law <law@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	rostedt@...dmis.org, David Daney <ddaney@...iumnetworks.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Haley <aph@...hat.com>,
	Richard Guenther <richard.guenther@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	feng.tang@...el.com, Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, jakub@...hat.com,
	gcc@....gnu.org
Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions

On 11/19/09 14:14, H. Peter Anvin wrote:
> Hence a new unconstrained option...
>    
Not arguing against it, just noting there are targets where after the 
prologue mcount is mandated.   There's certainly hooks in GCC to do it 
both ways and if there's no clear need to use after-prologue on 
x86-linux, then before-prologue seems reasonable to me.

It's also the case that aligning stacks on the x86 and the poor code 
generated when used with profiling is an interaction I doubt anyone has 
looked at until now.  The result is definitely ugly and inefficient -- 
and there's something to be said for cleaning that up and at least 
marginally reducing the overhead of profiling.

Having said all that, I don't expect to personally be looking at the 
problem, given the list of other codegen issues that need to be looked 
at (reload in particular), profiling/stack interactions would be around 
87 millionth on my list.

jeff

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